The unmanned spacecraft will travel to an asteroid near Earth called Bennu to collect space dust that may reveal how the materials necessary for life — such as carbon and ice — made their way to our planet.

"We seek samples that date back to the very dawn of our solar system," OSIRIS-REx principal investigator Dante Lauretta said.

Launching September 8 atop an Atlas V rocket from Cape Canaveral Air Force Station in Florida, the 2,087-kilogram four-wheel drive-sized spaceship is set to embark on a seven-year mission, returning its bounty of space dirt in 2023.

OSIRIS-REx builds on Hayabusa, Stardust missions

TAGSAM aims to return far more material than a similar mission by the Japanese Aerospace Exploration Agency that launched in 2003.

The Hayabusa mission encountered some difficulties, and even crash-landed into the surface of its target, but managed to return just under a milligram of material from the asteroid Itokawa in 2010.

NASA's spacecraft is designed to collect at least 60 grams, but could get much more. Tests have shown it picks up an average of 300 grams.

The sample will not be taken until July 2020, allowing the spacecraft time to closely study the asteroid so scientists can pick the most promising region, OSIRIS-REx program executive Gordon Johnston said.

"We will approach Bennu, map it, orbit it, study it and pick the safest and most scientifically interesting place to pick a sample from before going in and attempting the sample," he said.

"Three quarters of the sample will be set aside for future researchers — for the science questions we haven't figured out to even ask yet."

NASA has also promised 4 per cent of the sample to its major partner in the effort, Canada, and another 0.5 per cent to Japan.

Although the Japanese mission proved that asteroid sampling and return can be done, NASA based much of the OSIRIS-REx's design on a previous US mission launched in 1999, called Stardust, which returned cosmic material from a distant comet's tail.

"Stardust told us a lot about comets, which form way in the outer part of the solar system," Dr Lauretta said.

"OSIRIS-Rex is bringing back samples that are closer to home and are probably the likely candidates that delivered these key, prebiotic compounds to our Earth."

Bennu asteroid the 'right size'

NASA picked Bennu out of 500,000 known asteroids in the solar system because it is the right size, easy enough to visit, and contains carbon-rich dust from billions of years ago, OSIRIS-REx deputy program scientist Christina Richey said.

"We are talking about the start of the formation of our solar system," she said. "Maybe the precursors to life on Earth or elsewhere."

The space rock is taller than the Eiffel Tower — some 457 meters in diameter — and passes by Earth every six years, orbiting the sun at roughly the same distance as the Earth.

"Bennu is currently the best understood asteroid not yet visited by a spacecraft," Ms Richey said.